Data carriers are known in the form of smart cards, as emerges for example from DE 19634134 A1. In the case of the method described therein for transmitting data between a terminal and a portable data carrier via a wire-free electromagnetic transmission link, a 100% ASK modulation of the carrier signal takes place. This switching on/off of the carrier signal is comparatively straightforward to demodulate on a data carrier, but has the disadvantage that no clock signal is available during the blanking interval.
Besides the 100% ASK modulation, a 10% ASK modulation is also employed. In the case of this type of modulation, the carrier signal is not switched on and off, rather the amplitude of the carrier signal is merely reduced by 10%. Such a modulation is comparatively difficult to demodulate since, in the case of a change in the distance between a read/write device and the data carrier, the field strength that is effective there fluctuates greatly.
An improved data carrier is disclosed in EP 0940769 B1. The data carrier proposed therein is provided with a parallel regulator that keeps the voltage across the coupling element or at the output of the rectifier constant by controlling the current flowing through the parallel regulator. A current measuring device is provided between the rectifier and the parallel regulator and provides an input signal for an amplitude demodulator.
What is problematic about the known circuits is that data carriers have circuit components having a greatly fluctuating current consumption. There is the risk, therefore, that a detected current fluctuation will be interpreted as a field strength change in accordance with a 10% ASK modulation even though what is involved is a sudden change in load in one of the circuit components provided on the data carrier.
In order to avoid such errors, the circuit components of the data carrier can be operated in such a way that they do not generate any load changes. However, this is very complicated in the circuit design. U.S. Pat. No. 6,134,130 discloses providing a constant current sink which compensates for the irregular current demand of the chip. A uniform current demand then appears outwardly. What is unfavorable, however, is that the average current consumption rises, which also leads to an additional heating of the data carrier.